The electrophoretic mobility of haemoglobin was measured in a novel membrane-electrophoresis cell and by electrophoretic light scattering. The effect of protein concentration was investigated at different ionic strengths in two different buffer systems. The results indicated that although the effect of the concentration is weak, the mobility did decrease linearly with an increase in volume fraction throughout the range of volume fractions investigated (phi < 0.06). This dependence is more pronounced at lower kappa a values where double-layer interactions between the particles are more significant. Protein contribution to the solution ionic strength alone cannot explain the observed reduction in electrophoretic mobility. The theory of Shugai et al. (Shugai, A. A., Carnie, S. L., Chan, D. Y. C., and Anderson, J. L, J. Colloid Interface Sci. 191, 357 (1997)) was found to be adequate in describing particle interactions. The agreement with Shugai's theory is somewhat surprising considering the polydispersed nature of the samples, uncertainties in protein size, changes in ionic strength at high protein concentrations, and possible membrane-protein interactions not accounted for in the theory.